An examination of the interannual variations of tropical cyclone (TC) activity over the North Indian Ocean during 1983–2008 has been carried out. The results suggest that instead of local sea surface temperatures, such variations, at least over the Bay of Bengal (BB) during October-November-December (OND), can be attributed to similar variations in the atmospheric flow patterns and moist static energy that are apparently forced largely by the El Niño/Southern Oscillation (ENSO). In an El Niño year, conditions for TC genesis and development, including 850-hPa relative vorticity, 200–850-hPa vertical shear of zonal wind, moist static energy, 500-hPa zonal wind, 500-hPa and 850-hPa geopotential height and 200-hPa divergence, are generally less favourable in BB and fewer intense cyclones are observed during OND. The reverse occurs during a La Niña event. However, causes of the variability of TC activity over BB during April-May-June and that over Arabian Sea have yet to be found, which may be due to the small sample size. Copyright © 2011 Royal Meteorological Society